There are many types of small bodies in the solar system, but none may be as dramatic as comets. (Credit: National Science Foundation)

Some of the smallest things can have the biggest impacts. Some of the smallest bodies in our solar system have had the biggest impact on our understanding of how the solar system formed. Small bodies like comets, asteroids, and the small objects in the distant Kuiper Belt have been very difficult to see, much less study.

Join NASA's exploration of these curious members of our solar system and experience their impact -- on our understanding and on the planets. Make a model of an icy comet and create some impact craters! Other resources here will allow you to follow missions.

Comets, asteroids and Kuiper Belt objects remain the most difficult bodies to study within our solar system today. Scientists have proposed explanations about their evolution and composition. We are learning more about their role in the early solar system. (More information on the formation of the solar system is in our topic Birth of Worlds.)

Recently, scientists have begun to aggressively investigate both comets and asteroids, resulting in several NASA missions aimed at collecting data in coming years. In the past decade, NASA has studied three comets up close with the Deep Space 1 (Comet Borrelly), Stardust (Comet Wild 2) and Deep Impact (Comet Tempel 1) spacecrafts. In 2007, the Deep Impact and Stardust spacecrafts were found to be healthy and were retargeted to encounter new comets. Deep Impact's extended mission, EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation), encountered Comet Hartley 2 in November 2010. NASA returned to Comet Tempel 1 in February 2011, when the Stardust-NExT (New Exploration of Tempel 1) mission observed the changes in the comet nucleus since Deep Impact's 2005 encounter.

Small Bodies

There are many types of small bodies in the solar system, but none may be as dramatic as comets. The sight of a new, bright 'star' in the night sky -- one with a long tail or "beard" -- was frightening to ancient peoples. Any changes in the heavens were considered messages from the gods, portents of evil or omens of greatness. Even in 1910 there was wide spread panic that the world would end when Earth passed through the tail of Halley's Comet -- even though Halley's Comet was millions of kilometers from Earth. Astronomers had discovered evidence of cyanogens (cyanide-like molecules) in comets' tails. Newspapers predicted the end of the world. Gas mask vendors made big profits.

In fact, we do pass through the remnants of Halley's Comet dust tail every year. We experience this as a meteor shower called the Orionids in October. The Perseid meteor shower in mid-August occurs when Earth passes through the orbit of the Swift-Tuttle comet.

These beautiful visitors to the inner solar system probably come from the extreme outer parts of the solar system. Comets are huge 'snowballs' or 'icy dirt balls' of frozen gases, water ice, rock, and dust created when the solar system formed. The center, or nucleus, of a comet may range from 100 meters (300 ft) to 40 km (24 miles). When the comet is heated by solar radiation in the inner solar system, frozen gases in the comet turn to vapor and stream away from the comet nucleus. This creates a thin atmosphere, or coma, around the comet that can make the comet appear as a large glowing object with a tail millions of kilometers long.

Big Impacts

Comets and other small bodies have a big impact because they tell us so much about how our solar system formed. Comets tell scientists about conditions early in the formation of the solar system. Many comets come from the Kuiper Belt and the Oort Cloud. The Kuiper Belt is a large region that begins at Pluto and extends for billions of miles. The Oort Cloud is even further out. Scientists believe that icy and rocky objects in these regions far from the Sun are relatively unchanged since the beginning of the solar system. While it is difficult to get to these objects, some of the objects come to us. They come as comets that orbit close to the Sun at one end of their orbit and into the Kuiper Belt or Oort Cloud at the other end. The Stardust mission studied Comet Wild 2 because it was new to the inner part of the solar system and hadn't been changed by radiation from the Sun.

Comets also have a big influence when they collide with Earth. While collisions between Earth and comets are currently extremely rare, millions of years ago it was more common. Some scientists argue that colliding comets early in Earth's development contributed much of Earth's water. Some scientists also postulate that comets could have delivered organic molecules critical to the formation of life. Comets contain a lot of water and these special organic chemicals.